In many fluid systems it is essential to be able to switch a user back and forth between a pressure source and a sump. Such switching can be used, for example, to control an actuator or to vent excess pressure from a supply or reservoir. The device may be manually operated or may respond to a pressure threshold to switch from a state connecting the user to the pressure source to a state connecting it to the sump, or vice versa.
In the known systems the valve element moves between these two positions--sump-to-user and source-to-user--relatively sluggishly and, in an intermediate position, can actually connect user, source, and sump all together. Furthermore it is possible for the valve element to hang up in this intermediate position with possibly disastrous consequences for the units being controlled. In addition the valve element in these known devices is often a membrane that is flexed with each actuation so that after some time it wears out and must be replaced. Furthermore such valves are usually operated by a separate actuator to which they are connected via some sort of fluid line which can create problems with regard to response time and long-term durability.